A gamma ray scintillation type detector consists of a scintillation crystal optically coupled to a photomultiplier tube. Intensity of light induced within the crystal by an impinging gamma ray is proportional to the energy of the gamma ray. The optically coupled photomultiplier tube generates an electrical pulse which is proportional to the intensity of the light generated within the scintillation crystal. It follows, therefore, that the electrical pulse generated by the photomultiplier tube is proportional to the energy of the gamma ray impinging upon the scintillation crystal. These electrical pulses can be counted based on their energy level, which follows that such counts can be seen as a count rate of gamma rays at a particular energy level that have been detected by the scintillation crystal (or any other suitable gamma ray or photon detector). These counts can be grouped together to represent one or more count rates for particular energy ranges (or energy windows) of the collected gamma rays (or photons).
These count rates can be used to determine characteristics of material(s) surrounding the wellbore during logging operations. Logging wellbore characteristics during or after a wellbore has been formed provides valuable information about material(s) surrounding the wellbore, such as mud cake thickness and density, earth formation density, photo-electric factor of the material(s), etc. Logging can be done in various measurement while drilling MWD and/or logging while drilling LWD operations, as well as logging operations (e.g. wireline logging, coiled tubing logging, slickline logging, etc.) which can be independent of drilling operations. Current methods for determining wellbore characteristics can be seen as characterizing a measurement of a wellbore characteristic by correlating each energy-window count rate from multiple detectors to parameters of interest. More often than not, the mathematical representations of these correlations can be both complex and subject to undesirable statistical perturbations.
Therefore, it will be readily appreciated that improvements in the arts of using gamma ray count rates to determine wellbore characteristics are continually needed.